Next to naphthalene, phenanthrene is the second largest component of high temperature coal tar. It is concentrated in anthracene oil fraction (300-360° C.) of coal tar distillation. It constitutes 30-40% of the crude anthracene cake obtained from anthracene oil by cooling and cetrifuging. After recrystalisation of anthracene residue, phenanthrene is recovered from the filtrate by solvent extraction and/or fractional distillation. Phenanthrene and its derivatives, particularly 9:10-Phenanthraquinone, 2,2′-biphenyldicarboxylic acid (Diphenic acid) and 4,4′-biphenyldicarboxylic acid are in growing demand in the world market (annual growth rate 15%) due to their excellent performance in many newly developed applications, such as production of high temperature heat resistant resins, engineering plastics, liquid crystalline polymers, pharmaceuticals and agro-chemical industries etc.
Reference is made to U.S. Pat. No. 4,352,922 wherein basic chemistry of phenanthrene-derived poly-(amide-imide) resins is described. Although this method produces an acceptable poly-(amide-imide) resin with adequate thermal stability, the resin does not have very good impact resistance, tensile strength or elongation properties and cannot be drawn as long fibres. It is thought that the deficiencies in the physical properties of the resin are due to the low molecular weight of the phenanthrene/formaldehyde reaction product and the presence therein of many oligomers of phenanthrene having terminal moieties linked to the chain at either the 9 or 10 position. On oxidation, such a reaction product will give rise to diphenic acid residues which will act as chain terminators on reaction with the aromatic diamine. R. Behrend, Zeit. Phys. Chem., 1892, 9, p. 405; 10, p. 265 describes oxidation of phenanthrene by reacting alcoholic solution of phenanthrene with chromic acid, first to phenanthre-4uinone, and then to diphenic acid but the yield and purity are poor.
Reference made to U.S. Pat. No. 4,373,089 wherein phenanthrene is converted to its 9, 10 diol derivative via 9, 10 phenanthraquinone. The phenanthrene is oxidised by a mild oxidising agent, such as potassium dichromate, to produce the phenanthraquinone which is reduced to the 9, 10 phenanthrene diol by a mild reducing agent, such as sulphur dioxide. Sulphur dioxide is a convenient reducing agent because it is possible to bubble it through a solution of the phenanthraquinone to produce the 9, 10, diol derivative. The derivative can be protected from reoxidation by a blanket of an inert gas such as nitrogen. According to a second aspect of the present invention, there is provided a polyimide resin comprising the condensation product of a reaction between an aromatic diamine and a polycarboxylated product formed by reacting the 9, 10-diol derivative of a phenanthrene with formaldehyde and oxidising the reaction product to produce keto groups bridging the diphenic acid moieties produced. According to a third aspect of the present invention, there are provided intermediates in the formation of a polyimide resin comprising firstly the reaction product of a 9, 10-diol derivative of a phenanthrene with formaldehyde in the presence of an acid catalyst, and secondly the reaction product which has been oxidised to produce keto groups bridging the diphenic acid moieties produced.
Prior art search for production of diphenic acid was done based on literature survey and patent databases and did not yield any relevant references.